Dark Matter Made Visible

I've been chasing the elusive dark matter story in astronomy for some twenty-seven years now, ever since I did an article on the "missing mass" in the (now defunct) Science Digest magazine in 1983.  I followed up with a book, Through a Universe Darkly, a decade later.  I haven't tired of the subject and still avidly follow each new clue that appears on the scene. It's mind-boggling that around 85 percent of the matter in the universe is in a form that remains invisible.

The latest evidence comes from the Hubble Space Telescope (HST).  A team of astronomers led by Dan Coe of NASA's Jet Propulsion Laboratory recently aimed a HST camera at a massive galaxy cluster known as Abell 1689, located more than two billion light-years away.  While astronomers can't see dark matter, they can certainly view its gravitational effects, especially in the way dark matter diverts light passing through the universe.  You might think of the dark matter in the cluster as a massive stone with the light passing by acting like a stream of water that comes upon the rock and is split into two or more streams.  In other words, dark matter bends light arriving from the distant galaxies behind it, much like a lens (which is why the effect is called "gravitational lensing"). What results is a funhouse view of the cluster, filled with myriad arcs, bands, and rings of light.  The amount of light bending provides the means to "weigh" the dark matter in the cluster.

This is not a new approach.  Abell 1689, due to its rich collection of galaxies, has been studied for years in this way.  But this latest image is the highest resolution yet, which enabled the astronomers to map the dark matter's distribution within the cluster. 

Galaxy Cluster Abell 1689, located 2.2 billion light-years distant

Each spot in this picture is a far-off galaxy, not a star.  The Abell 1689 cluster contains about 1,000 galaxies in all.  The light blue haze has been artificially added to indicate where the dark matter is concentrated within the cluster. 
Image Credit: NASA, ESA, and Z. Levay (STScI)